The non-patent document 1, which is recited below, discloses a result of evaluation of dry friction characteristics evaluated at the time of contacting, i.e., coupling a disk test piece, which is made of a ferroalloy, and a ring test piece together. In the disk test piece used in this evaluation, iron oxide powder is embedded in an inside of a recess formed in a contact surface of the disk test piece. According to this result of the evaluation, when coupling and decoupling between the disk test piece and the ring test piece are executed while rotating the disk test piece, a friction coefficient between the disk test piece and the ring test piece becomes high and is stabilized.